Process employing thin-walled, extruded activated carbon filter

ABSTRACT

An extruded, thin-walled activated carbon filter tube having a wall thickness of about 0.5 inch (12.7 mm) or less and an average dry density sufficient to remove at least about 99.5% of sub-micron particles from a liquid filtered through said tube.

RELATED APPLICATION

This application is a divisional of application Ser. No. 08/902,571,filed Jul. 29, 1997, now U.S. Pat. No. 5,922,803, which is acontinuation of application Ser. No. 08/607,294, filed Feb. 26, 1997,now abandoned.

FIELD OF THE INVENTION

This invention relates to a filter process employing a novelthin-walled, extruded activated carbon filter, especially such a filterfor use in room temperature water purification processes for removal ofsub-micron sized particulates.

BACKGROUND OF THE INVENTION

Presently used in filtration systems, where particulate removal in therange of 99.5% or more of particulates of a size of 0.5 μm to 0.8 μm,are ceramic candle filters. Such ceramic filters are primarily designedfor high temperature applications. However, due to the unavailability ofsuitable lower cost filters which would still provide the aforesaiddegree of sub-micron particulate removal, such ceramic filters have alsobeen employed for filtration or purification processes even when hightemperatures are not required, such as at room temperature processes forpurification of water. Such use of ceramic filters, when their hightemperature properties are not required, results in a greatly increasedfiltration or purification cost due to the costly nature of such ceramicfilters.

Although activated carbon filters have been known and used in waterpurification processes, it has not heretofore been possible to achievethe aforesaid level of particulate removal employing known activatedcarbon filters.

It would therefore be desirable that a highly effective thin-walledactivated carbon filter be available for use in low temperaturefiltration and purification processes to replace the unduly expensiveceramic filter units now employed to obtain 99.5% or more removal ofparticulates of a size of 0.5 μm to 0.8 μm.

SUMMARY OF THE INVENTION

It has been discovered that an extruded, thin-walled activated carbonfilter tube capable of removing 99.5% or more of sub-micron sizedparticles of a size of from 0.5 μm to 0.8 μm from water or other liquidsat room temperature is provided according to this invention by extrudingsuch a thin-walled activated carbon filter from a composition comprisingan essentially homogeneous admixture of:

(a) from about 75% to about 85% by weight activated carbon particles,and

(b) from about 15% to about 25% by weight of a thermoplastic binderpowder having an average particle size of from about 5 μm to about 25μm;

wherein the activated carbon particles comprise:

(a) from about 18% to about 50% by weight of activated carbon particlespassing through a 325 mesh screen, i.e. −325 mesh fraction, and

(b) from about 50% to about 82% by weight of activated carbon particlespassing through a 200 mesh screen and retained on a 325 mesh screen,i.e. −200 to +325 mesh fraction.

By thin-walled filter tube of this invention it is meant that theactivated carbon filter tube has a wall thickness of 0.5 inch (12.7 mm)or less, more preferably 0.3 inch (7.62 mm) or less.

A thin-walled activated carbon filter can be extruded from saidcomposition by the continuous extrusion process disclosed in my earlierU.S. Pat. No. 5,189,092, and will have a density sufficient to remove99.5% of particles of a size of 3 microns or larger, preferablysufficient to remove 99.5% of particles of a size of about 0.5 μm to 0.8μm.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a cross-sectional view of an activated carbon filter ofthis invention.

DETAILED DESCRIPTION OF THE INVENTION

Although my earlier U.S. Pat. No. 5,189,092 disclosed extrusion ofcompositions comprising activated carbon particles and thermoplasticbinder powder to produce filtration articles, the activated carbonfilters produced in said patent were not capable of particulate removalof 99.5% or more of particles of a size of 0.5 μm to 0.8 μm, nor was itconsidered possible to produce activated carbon tubes having a wallthickness of less than 0.5 inch (12.7 mm) and especially such tubeshaving a wall thickness of 0.3 inch (7.62 mm) or less.

It has been discovered that with the aforedescribed homogenous mixtureof the two different sized activated carbon particles, i.e. from about75% to about 85% by weight of about 18% to 50% by weight of −325 meshactivated carbon particles and about 50% to 82% by weight −200 to +325activated carbon particles, employed with about 15% to about 25% byweight of thermoplastic powder having an average particle size of fromabout 5 μm to about 25 μm, extrusion of such a thin-walled filter tubecould be accomplished to provide a filter element capable of removing99.5% or more of particles of a size of 0.5 μm to 0.8 μm.

Any suitable activated carbon particles of the stated sizes can beemployed in producing the thin-walled filter tubes of this invention.Among those activated carbon particles suitable for use and availablefrom Calgon Carbon Corporation is Type TOG-NDS activated carbon, whichis understood to be acid washed, bituminous coal-based activated carbonparticles.

Although from about 18% to about 50% by weight of −325 mesh activatedcarbon particles may be employed, it is preferred that the amount ofsaid −325 mesh particles be about 28% by weight of the activated carbonparticles and that 72% by weight of the activated carbon particles bethe −200 to +325 mesh particles. Also, it is preferred that the amountof activated carbon particles comprise about 81% by weight of theessentially homogeneous mixture and the binder powder comprise about 19%by weight of said mixture.

The thermoplastic binder can be composed of nearly any thermoplasticmaterial including, for example, polyolefins such as polyethylene,polypropylene, polybutene-1 and poly-4-methyl-pentene-1; polyvinyls suchas polyvinyl chloride, polyvinyl fluoride and polyvinylidene chloride;polyvinyl esters such as polyvinyl acetate, polyvinyl propionate andpolyvinyl pyrrolidone; polyvinyl ethers; polyvinyl sulfates; polyvinylphosphates; polyvinyl amines; polyoxidiazoles; polytriazols;polycarbodiimides; copolymers and block interpolymers such asethylene-vinyl acetate copolymers; polysulfones; polycarbonates;polyethers such as polyethylene oxide, polymethylene oxide andpolypropylene oxide; polyarylene oxides; polyesters, includingpolyarylates such as polyethylene terphthalate, nylons polyimides andvariation on these and other polymers having substituted groups such ashydroxyl, halogen, lower alkyl groups, lower alkoxy groups, monocyclicaryl groups and the like, and other thermoplastic meltable solidmaterials. Preferred are polyethylene, poly(ethylene vinyl acetate) andnylons. Especially preferred as a binder is polyethylene, particularlygrade SN 510 polyethylene from the USI Division of Quantum ChemicalCorporation.

If desired, a small amount of the about 75% to about 85% by weightactivated carbon particles in the essentially homogeneous mixture can bereplaced with additional particulate or powdered additive materials toenhance the performance of the filter. For example, from about 6% to 20%by weight of zeolite can be added to increase adsorption of heavymetals, such as lead, from the water or other liquid to be filtered. Anespecially preferred zeolite that can be employed as such an additive isATS titanium silicate zeolite with high specificity for adsorbing lead,which zèolite is available from Engelhard Corporation. As examples ofother additives which may be employed in small amounts, generally inamounts of less than about 10% by weight, there may be mentioneddiatomaceous earth, ceramics, ion exchange resins and the like.

The components of the extrudable composition are to be mixed to anessentially homogeneous mixture before extrusion of the mixture into thedesired thin-walled carbon tubes. After the components have been mixedto an essentially homogeneous mixture, the mixture can be extruded intothe desired thin-walled carbon tubes by extrusion according to theprocess and with the apparatus described in my U.S. Pat. No. 5,189,092.

The extruded, thin-walled carbon tubes of this invention will generallyhave a density of from about 0.65 to about 0.75 g/cm³. For example,according to this invention it is possible to produce an activatedcarbon filter tube of an average density of 0.72 g/cm³ having a 2.0 inch(50.8 mm) O.D. and a 1.4 inch (35.56 mm) I.D., i.e. a wall thickness of0.3 inch (7.62 mm). The tube can be continuously extruded and cut to anydesired length, such as for example, 10 inches long (254 mm).

In accordance with this invention such an extruded thin-walled activatedcarbon tube is produced according to U.S. Pat. No. 5,189,092, cut to thedesired length and then the tube is mounted or bonded to two suitableend caps or to one end cap and mounted in a suitable cartridge housingor some similar suitable configuration for filters.

Illustrated in the FIGURE is a typical thin-walled filter tube elementof this invention mounted in a cartridge and suitable for use as a unitin purifying water to sub-micron particle size levels. It is to berecognized that the thin-walled filter tubes of this invention can beused in any suitable environment and that its description of beinghoused in the cartridge of the FIGURE is merely an exemplary andnon-limiting embodiment.

The filtration unit 10 of the FIGURE comprises a cartridge 12 housing athin-walled, extruded filter tube 14 of this invention. The cartridge 12can be made of any suitable material, such as for example, metal orplastic, such as polypropylene or the like. The cartridge 12 comprises asubstantially cylindrical hollow housing having sidewall 16, which at afirst end 18, such as a lower end, can be slightly flared to fit into asuitable base for mounting the filter unit 10 to a source of water orliquid to be purified. The lower end 18 is closed by a base wall 20. Thebase 20 includes an axial passageway 22 to the interior of the hollowcartridge 12. A conduit or pipe 24 is mounted in said axial passageway22 for permitting filtered, purified water or other liquid to exit fromthe interior of the hollow cartridge 12. If desired or necessary, thesidewall 16 can be provided with an exterior groove 26 with an O-ring 28mounted therein for sealingly sealing the filtration unit 10 in a wateror liquid supply source (not shown). A radial passageway 30 is providedin the sidewall for permitting water or other liquid to be filtered toenter into the interior of the hollow cartridge. A thin-walled,extruded, porous activated carbon filter tube 14 of this invention isprovided with a capping disk 32 at one end to close said end of thefilter tube. The disk 32 has a central protrusion 34 which extends intothe hollow tubular passageway 36 of the filter tube 14 for purposes ofalignment. The disk 32 is also provided with a peripheral rim 38 foraligning the filter tube 14 and closing off the end of said filter tube.The function of disk 32 is to prevent water or other liquid from passingaxially into the hollow tubular axial passageway 36 of the filter tube14.

At the second or upper end of the sidewall 16 the cartridge 12 is closedwith an end cap 40 provided with a central recess 42 for receiving,holding and aligning capping disk 32 by engaging a raised central hub 44on the disk. The disk 32 may be provided with a plurality of radial ribs46 which can also assist in the alignment process.

The thin-walled filter tube 14, provided with capping disk 32 at one endand the filter tube, is inserted into cartridge 12 with the open end ofthe filter tube surrounding axially passageway 22 and exit conduit 24.Subsequently, the cartridge is closed by end cap 40 by inserting raisedhub 44 on disk 32 into the central recess 42 of the end cap. Theassembled filtration unit 10 can now be employed by installation into asystem whereby water or other liquid to be purified enters cartridge 12through radial passageway 30 and flows into the interior space 48between the cartridge sidewall 16 and the thin-walled filter unit 14.Because the upper end of the tubular axial passageway 36 is blocked bycapping disk 32, the water or liquid is forced to flow radially throughthe porous thin-walled filter tube and down the tubular axial passagewaywhere it enters the conduit 24 at the bottom of the cartridge 12,flowing therethrough as water or other liquid purified to a sub-micronlevel.

The invention is further illustrated by the following example of thepreparation and testing of a thin-walled activated carbon filter tube ofthis invention.

EXAMPLE

A substantially homogeneous mixture is prepared by mixing about 81% byweight of type TOG-NDS acid washed, bituminous coal-based activatedcarbon particles with about 19% by weight type SN 510 polyethylenepowder having a particle size within the range of from 5 μm to 25 μm.The 81% by weight activated carbon comprised about 28% by weight of −325mesh activated carbon particles and about 72% by weight of −200 to +325mesh activated carbon particles. The substantially homogeneous mixturewas then extruded into a thin-walled hollow tube of 2″ O.D. (50.8mm)×1.4″ I.D. (35.56) and having an average dry density of about 0.72g/cm³ by the extrusion process and apparatus disclosed in theaforementioned U.S. Pat. No. 5,189,092. The substantially homogeneousmixture was heated for about 5 minutes in the extruder heating zonemaintained at a temperature of about 375° F. (190° C.), and cooled forabout 3 minutes in the extruder cooling zone maintained at a temperatureof about 95° F. (35° C.) before being extruded as a thin-walled hollowtube. The thin-walled hollow tube was then cut into suitable lengths ofabout 10 inches (254 mm) for use in testing as a filter for removingsub-micron sized particles.

Using water contaminated with PTI fine test dust powder, the filtertubes of this invention were tested for sub-micron sized particulateremoval. Flowing such contaminated water at 0.5 gal/min. through theabove produced thin-walled filter tubes mounted in an appropriatecartridge and having a pressure drop across the filter of about 14psi_(d) resulted in the exiting purified water having a 99.9% reductionof contaminating dust particles of a size of from 0.5 μm to 0.8 μm asmeasured using a laser in-line particle counting technique.

When a similar type thin-walled filter tube is produced having anaverage dry density of 0.67 g/cm³ and tested with said dust contaminatedwater flowing at 0.5 gal/min. with a pressure drop across the filter of7.5 psi_(d), 99.8% of particles in the size range of from 0.8 μm to 1 μmwere removed as measured by the laser in-line particle countingtechnique.

The extruded thin-walled activated carbon filter tubes of this inventionare characterized by the fact that the surface of the extruded tubes areextremely hard and smooth and as a result particulates filtered out onthe outer surface of the extruded tubes are readily removed which permitrepeated re-use of the filter tubes after cleaning. Additionally, thehard/smooth surface of the extruded thin-walled activated carbon filtertubes of this invention prevents release of carbon particles from theinterior surface of the tubular filters and into the purified water orother liquid.

With the foregoing description of the invention, those skilled in theart will appreciate that modifications may be made to the inventionwithout departing from the spirit thereof. Therefore, it is not intendedthat the scope of the invention be limited to the specific embodimentsillustrated and described.

What is claimed is:
 1. A process for purifying water or other filterableliquid to remove at least 99.5% of sub-micron particles from said wateror other filterable liquid comprising filtering the water or otherfilterable liquid through the wall of an extruded, thin-walled activatedcarbon filter tube which has been extruded from a composition comprisingan essentially homogeneous admixture of; (a) from about 75% to about 85%by weight activated carbon particles, and (b) from about 15% to about25% by weight of a thermoplastic binder powder having an averageparticle size of from about 5 μm to about 25 μm; wherein the activatedcarbon particles comprise; (a) from about 18% to about 50% by weight ofactivated carbon particles passing through a 325 mesh screen, and (b)from about 50% to about 82% by weight of activated carbon particlespassing through a 200 mesh screen and retained on a 325 mesh screen. 2.The process of claim 1 comprising the purifying of water.
 3. A processfor purifying water or other filterable liquid to remove at least 99.5%of sub-micron particles from said water or other filterable liquidcomprising filtering the water or other filterable liquid through thewall of an extruded, thin-walled activated carbon filter tube which hasbeen extruded from a composition comprising an essentially homogeneousadmixture of: (a) about 81% by weight activated carbon particles, and(b) about 19% by weight of a thermoplastic binder powder having anaverage particle size of from about 5 μm to about 25 μm; and wherein theactivated carbon particles comprise about 28% by weight of activatedcarbon particles passing through a 325 mesh screen and about 72% byweight of activated carbon particles that pass through a 200 mesh screenbut are retained on a 325 mesh screen.
 4. The process of claim 3comprising the purifying of water.
 5. A process for purifying water orother filterable liquid to remove at least 99.5% of sub-micron particlesfrom said water or other filterable liquid comprising filtering thewater or other filterable liquid through the wall of a thin-walledactivated carbon filter tube having a wall thickness of about 0.5 inch(12.7 mm) or less and an average dry density from about 0.65 to about0.75 g/cm³ wherein the tube has been extruded from a compositioncomprising an essentially homogeneous admixture of: (a) about 81% byweight acid-washed, bituminous coal-based activated carbon particles,and (b) about 19% by weight of a thermoplastic binder powder having anaverage particle size of from about 5 μm to about 25 μm; wherein theactivated carbon particles comprise: (a) about 28% by weight ofactivated carbon particles passing through a 325 mesh screen, and (b)about 72% by weight of activated carbon particles that pass through a200 mesh screen but are retained on a 325 mesh screen.
 6. The process ofclaim 5 comprising the purifying of water.